The present invention broadly relates to improved electrical energy storage systems, and more particularly the invention relates to a novel method of removing inert gases from a metal halogen battery system.
The electrical energy storage systems of the type referred to herein (e.g., a zinc-chlorine battery) utilize a halogen hydrate as the source of a halogen component for reduction at a normally positive electrode, and an oxidizable metal adapted to become oxidized at a normally negative electrode during the normal discharge of the storage system. An aqueous electrolyte is employed for replenishing the supply of the halogen component as it becomes reduced at the positive electrode. The electrolyte contains the dissolved ions of the oxidized metal and the reduced halogen, and is circulated between the electrode area and a storage area containing halogen hydrate, which progressively decomposes during a normal discharge of the electrical energy system, liberating additional elemental halogen to be consumed at the positive electrode. Electrical energy storage systems or battery systems of this type are described in prior patents owned by the same assignee as the present invention, such as U.S. Pat. No. 3,713,888, U.S. Pat. No. 3,993,502, U.S. Pat. No. 4,001,036, and U.S. Pat. No. 4,146,680. Such systems are also described in published reports prepared by the assignee herein, such as EPRI Report EM-1051 (Parts 1-3) dated April 1979, published by the Electric Power Research Institute. The specific teachings of the aforementioned U.S. patents and the EPRI Report are incorporated herein by reference.
In the operation of a metal halogen battery, such as an aqueous zinc-chloride battery system with graphite and/or other stable electrode substrates, a finite proportion of gases other than the desired chlorine are evolved. With graphite electrodes, these other gases typically consist of carbon dioxide, carbon monoxide, and oxygen and hydrogen. Because of the acid electrolyte used in such battery systems, hydrogen is the major foreign gas liberated. In the past, photochemical techniques such as those described by Fatica in U.S. Pat. No. 4,144,381 have been utilized to recycle hydrogen to the electrolyte as hydrogen chloride. It is also to be recognized that the removal of carbon dioxide, carbon monoxide, and oxygen from the system will lead to slow acidification of the battery, therefore, from time to time hydrogen should be removed from such a battery system along with the other species so as to balance the pH of the battery. Slow accumulation of carbon dioxide, carbon monoxide, and oxygen causes a decrease in both chlorine hydrate formation ability and the solubility of chlorine in the electrolyte for discharge, thereby resulting in a decreased battery performance.
Several techniques for removing inert gases (i.e., inert gases being defined herein as gases other than chlorine), while maintaining the chlorine balance of the system have been disclosed. For example, see Bjorkman U.S. Pat. No. 4,256,554, Fatica U.S. Pat. No. 4,144,381, and Hammond et al U.S. Pat. No. 4,257,867. Those techniques typically involved a chlorine/chlorine electro-winning cell as the means of effecting the separation and elimination of the inert gases. It is the object of the present invention to provide a different, unique, and yet simple technique of removing inert gases from metal halogen battery systems. Other objects, features and advantages of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawing. It is also to be understood that the invention herein is applicable to numerous different constructional arrangements of metal halogen battery systems.